Dolphin use a form of sonar to locate food fish, such as mullet, and, it is believed, that tuna travel with dolphin because the tuna does not have sonar ability. In the past, there have been several techniques utilized by commercial tuna fishermen to separate dolphin from tuna. In one technique, tuna fishermen in small boats in advance of the nets have attempted to chase the dolphin away from the nets and this technique has resulted in the injury and possible loss of life to tuna fishermen and still results in some dolphin being caught in tuna nets. Others have attempted explosions in the water to frighten the dolphin but this can be harmful to dolphin in that it can cause deafness.
Dolphin, when properly stimulated, can swim much faster than tuna. The locomotion of dolphins is typical of the whale. The main thrust comes from vertical oscillations of the tail flukes, and most species are capable of sustained swimming speeds of up to 30 km/h (18.6 mph). In shorter bursts, dolphins can attain a speed of 37 to 40 km/h (23 to 25 mph), and in a few instances a speed of 48 km/h (30 mph) has been reported. Shipboard observers commonly see dolphins swimming in the bow wave of a moving vessel. The animals actually ride the bow wave by using the thrust of the ship--they seem to rest motionless as they are carried along. Tuna, on the other hand are a school fish and travel much more slowly. Even when agitated, they generally stay within the school envelope which moves rather slowly. The Parra invention utilizes this fact and the primordial or hereditary fear by dolphin of the killer whale to physically separate dolphin from tuna. This permits the tuna to be caught in tuna nets without the dolphin being captured in the tuna net. According to the invention disclosed in the above-identified Parra application, the sound of one or more killer whale feeding is captured by a transducer, converted to digital signals and then edited to edit out splashing sounds or other non-useful portions of the recording. In addition, the killer whale feeding sounds are enhanced by producing more of them. Thus, the killer whale's feeding frenzy sounds are recorded, digitized, edited and enhanced and then broadcast in the sea water by a high power acoustic transducer to simulate the killer whale feeding sounds.
The dolphin, upon hearing the sound rapidly swim away from the source. The tuna, which do not have the "memory factor" of the dolphin, may try to keep up but, because they have a blimpish body and are a "school fish", while individual tuna may be fast, as a school they are slower (flank speed of a tuna school is about 3.33 km/h (2 mph) than the dolphin stimulated by the feeding killer whale sounds and are quickly isolated or separated so the tuna may be caught in nets without harm to the dolphin. In the preferred embodiment, these enhanced feeding sounds of killer whales are digitally stored and selectively reproduced in one or several of a plurality of selectively dispersed underwater transducers. In one embodiment, the transducers are dispersed by small boats or a helicopter or other aircraft in a predetermined pattern and in an area between or behind the fishing nets relative to a mixed school of tuna and dolphin. Each transducer is selectively controlled by a radio signal and the movement of the dolphin observed. Should the dolphin head in a direction which would lead them to danger of the nets, a transducer in that area is activated to generate the killer whale feeding sound and thereby steer the dolphin toward a safe area. In one embodiment, the transducers have floats and an associated recovery transponder loop or hook so that they may be easily retrieved.